The following disclosure is based on German Patent Application No. 10211611.3, filed on Mar. 12, 2002, which is incorporated into this application by reference.
The invention relates to a method and a device for decontaminating the surfaces of beam-guiding optics employing UV-radiation in a cleansing atmosphere. Methods and devices of this type are used for, for example, decontaminating the surfaces of the lenses and other optical elements of beam-guiding optics, such as those employed on microlithographic projection-exposure systems.
In the case of modern microlithographic projection-exposure systems that employ very-short-wavelength radiation from the deep-ultraviolet (DUV) spectral region having wavelengths ranging from about 100 nm to about 300 nm for their operation, especially those that employ, e.g., 157-nm laser radiation for their operation, contaminants present on the surfaces of their beam-guiding optics that are struck by illuminating radiation cause readily noticeable, serious, problems due to absorption and/or scattering. The decontamination process is intended to remove such contaminants, or at least reduce their presence to acceptable levels.
A decontamination method for that purpose and an associated decontamination device incorporated into a microlithographic projection-exposure system are described in German patent application DE 198 30 438 A1. In the case of the technology employed therein, in addition to a DUV excimer laser serving as an exposure source, a second UV light source, e.g., a broadband DUV excimer laser or a 222-nm UV excimer lamp, serving as a decontaminating light source is also provided. The decontaminating light source incorporated into the projection-exposure system is activated during pauses in the employment of the exposure light source. In parallel therewith, a stream of cleansing gas is directed at the surfaces to be cleaned, where employment of a stream of gas containing ozone or oxygen was proposed. The latter is to be interpreted as a stream of gas having an oxygen content exceeding that of air, since that particular approach is based on the presumption that obtaining an adequate cleansing effect requires a correspondingly high oxygen concentration in the stream of cleansing gas.
This cleansing effect employing a combination of UV-radiation and an oxygen-rich or ozone-rich gas is also known in the case of decontamination of the surfaces of substrates, such as the surfaces of glass substrates and wafers. Refer to, for example, Japanese patent application JP 07-288109 A, where the combination of Xe-excimer-laser radiation at a wavelength of, e.g., 172 nm, with a cleansing atmosphere containing ozone or an air atmosphere was proposed. The purpose of the oxygen is oxidizing organic contaminants and forming hydrophilic groups in the concentrations regarded as necessary. The fact that the range of the UV-radiation at wavelengths of about 172 nm employed for cleansing purposes is relatively short due to high absorption by oxygen is counter-acted by moving the surfaces to be cleaned sufficiently close, e.g., to within less than 3 mm, to the UV light source employed for cleaning or employing longer-wavelength UV-radiation, e.g., UV-radiation having wavelengths of 185 nm or 254 nm, for cleansing purposes.
Although cleaning the surfaces of beam-guiding optics that have been designed for guiding relatively short-wavelength laser radiation having wavelengths of, e.g., 157 nm, using laser radiation having the same wavelength is feasible, in practice, it is a very difficult procedure and confined to relatively small areas covering just a few square millimeters of the surfaces to be cleaned.
One technical problem addressed by the invention is providing a method and a device of the type mentioned at the outset hereof with which the surfaces of beam-guiding optics, in particular, optics for guiding the beams of DUV-lasers emitting at wavelengths of about 157 nm and less, with relatively little effort and reliably decontaminating surfaces bearing disturbing contaminants, even contaminants that are distributed over large areas thereof.
The invention solves that problem in one aspect by providing a method for decontaminating surfaces of beam-guiding optics, in particular, the surfaces of optical elements of microlithographic projection-exposure systems, where the surfaces to be cleaned are irradiated with UV-radiation in a cleansing atmosphere, wherein the wavelength of the UV-radiation employed for cleaning falls within a spectral range where oxygen absorbs strongly and the cleansing atmosphere has an oxygen concentration less than that of air. In a further aspect the invention provides a device for decontaminating surfaces of beam-guiding optics, in particular, the surfaces of optical elements of microlithographic projection-exposure systems, having one or more UV-radiation sources for generating UV-radiation for cleaning purposes and means for providing a predeterminable cleansing atmosphere, wherein the wavelength of the UV-radiation employed for cleaning falls within a spectral range where oxygen absorbs strongly and the means for providing the cleansing atmosphere is designed to provide a cleansing atmosphere that has an oxygen concentration less than that of air.
According to the invention, UV-radiation having a wavelength falling within a spectral range characterized by strong absorption by oxygen is provided for decontamination purposes and the problem of the decontaminating radiation having a short range due to the high absorption by oxygen is counteracted by utilizing a cleansing atmosphere that has a comparatively low oxygen concentration that is less than that of air. Investigations have shown that combining such decontaminating radiation with a cleansing atmosphere having a low oxygen content, or no oxygen content, will allow attaining adequate cleaning actions, even in the case of beam-guiding optics that have been designed for guiding short-wavelength UV laser light having a wavelength of, e.g., 157 nm, such as those employed on microlithographic projection-exposure systems that employ DUV-radiation for their operation.
In a preferred embodiment of the invention, the oxygen concentration in the cleansing atmosphere is held to less than 1%, preferably less than 0.1%.
In a beneficial embodiment of the invention, the decontaminating radiation is generated by Xe-discharge lamp having a wavelength of 172 nm; or a low-pressure mercury lamp. In either case, the effort required to achieve adequate cleansing actions is much less than when short-wavelength laser radiation having a wavelength of about 157 nm is employed.
An advantageous embodiment of a decontamination device comprises a cleaning chamber having a cleaning section, into which, e.g., optical elements, may be placed for the purpose of cleaning their surfaces. Several UV-radiators that are spaced at intervals and act on the cleaning section are employed for cleaning, where a stream of cleansing gas having a low oxygen content, or no oxygen content, may be conducted over the full extent of the cleaning section.
In a further beneficial improvement on the invention, the decontamination device is incorporated into an optical module in order that the surfaces of its optical elements may also be cleaned from time to time after the module has been placed in service. The optical module may be part of a microlithographic projection-exposure system in order that the surfaces of its optical elements may be cleaned whenever necessary.